1. Field of the Invention
The invention is related to the field of holographic methods for recording fast movies whose speed is limited by the laser pulse duration if the recording material has sufficient sensitivity to reliably record a frame of the fast event with a single pulse.
2. Description of the Prior Art
Monitoring fast phenomena is of interest to science and engineering since it provides data about the dynamics of physical processes. For instance, the pump-probe technique is widely used in nondestructive and repeatable measurements. For one dimensional imaging (e.g. lifetime measurements), streak cameras with subpicosecond resolution can be used. Recording movies of fast events can be accomplished with a set of sensors. The light from the object is gated electronically on to each sensor (intensified CCD) while the image is broadcasted. Limited by weak intensity and silicon circuit speed, about 10 frames can be recorded with the time resolution of 10 ns. Since the early days when holography was invented, people have been studying high speed events using holographic techniques. A well known example is double exposure interferometry. Multiple frames can be stored and reconstructed separately using multiplexing techniques. Previous work has focused on spatial multiplexing where holograms are recorded at different locations of the recording medium. In an early report, a 100 ps laser pulse is used to store five frames. In another method, a wave front preserving optical delay line (a White cell) and a specially graded beam splitter is used to generate the reference and signal pulse trains. The frame interval can be as short as 28.3 ns. Spatial multiplexing by rotating the recording medium has also been report. Pulse holograms have also been angularly multiplexed taking advantage of the thickness of the recording medium. In one method, three lasers are used to generate three reference beams with different angles and each laser fires a pulse in a different time. The frame interval is about 1 μs. A rotating mirror or electro-optic switches have also been used to generate the reference beams. In these efforts, the speed is limited by electronics or mechanical scanning.
In spatial multiplexing, the number of frames is limited by the recording medium area and the beam diameter, not the dynamic range of the recording material. Furthermore, in general the direction of signal beam changes from pulse to pulse (except if the recording medium is rotated) and thus it changes the perspective of the object. It also makes the interferometry between any two of the recorded frames more difficult.